1. Field of the Invention
The present invention relates to a scintillator single crystal used in a single-crystal scintillation detector (scintillator) for gamma ray or other radiation in the fields of radiology, physics, physiology, chemistry, mineralogy and oil exploration, such as for medical diagnostic positron CT (PET), cosmic radiation observation, underground resource exploration and the like, as well as to a heat treatment method for production of the scintillator single crystal and to a method for production of the scintillator single crystal, and more specifically, the invention relates to a scintillator single crystal containing a cerium-activated orthosilicate compound, to a heat treatment method for production of the scintillator single crystal and to a method for production of the scintillator single crystal.
2. Related Background Art
Scintillators composed of cerium-activated gadolinium orthosilicate compounds have short scintillation decay times and large radiation absorption coefficients, and are therefore employed as radiation detectors for positron CT and the like. However, while the light output of such a scintillator is larger than that of a BGO scintillator, but is only about 20% of the light output of a NaI (Tl) scintillator and is therefore in need of further improvement.
Scintillators employing single crystals of cerium-activated lutetium orthosilicates represented by the general formula Lu2(1−x)Ce2xSiO5 (see Japanese Patent Publication No. 2852944 and U.S. Pat. No. 4,958,080), scintillators employing single crystals of compounds represented by the general formula Gd2−(x+y)LnxCeySiO5 (where Ln is at least one element selected from the group consisting of Sc, Tb, Dy, Ho, Er, Tm, Yb and Lu) (see Japanese Examined Patent Publication HEI No. 7-78215 and U.S. Pat. No. 5,264,154), and scintillators employing single crystals of cerium-activated lutetium yttrium orthosilicate represented by the general formula Ce2x(Lu1−yYy)2(1−x)SiO5 (see U.S. Pat. No. 6,624,420 and U.S. Pat. No. 6,921,901) have become known in recent years. Not only are such scintillators known to have improved crystal density, but cerium-activated orthosilicate compound-containing single crystals are known to have higher light outputs and shorter scintillation decay times.
In addition, Japanese Patent Publication No. 3668755 and U.S. Pat. No. 6,278,832 describe scintillation materials based on silicate crystals comprising lutetium (Lu) and cerium (Ce), including an oxygen vacancy a and having a chemical composition represented by the general formula: Lu1−yMeyA1−xCexSiO5−zαz, where:
x=1×10−4˜0.2
y=1×10−5˜0.05 [wherein A is at least one element selected from the group consisting of Lu and Gd, Sc, Y, La, Pr, Nd, Sm, Eu, Tb, Dy, Ho, Er, Tm and Yb, and Me is at least one element selected from the group consisting of H, Li, Be, B, C, N, Na, Mg, Al, P, S, Cl, K, Ca, Ti, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Ga, Ge, As, Se, Rb, Sr, Zr, Nb, Mo, Ru, Rh, Pd, Ag, Cd, In, Sn, Sb, Cs, Ba, Hf, Ta, W, Re, Os, Ir, Pt, Au, Hg, Tl, Pb, Bi, U and Th]. Japanese Patent Publication No. 3668755 and U.S. Pat. No. 6,278,832 mention over 50 elements from H to Th as the element Me instead of Lu, and such elements are described as having effects of rendering the crystals resistant to cracking during the cutting and production of scintillation elements, as well as effects of bringing out the waveguide properties in waveguide elements.
It is further mentioned that including ions with oxidation numbers of +4, +5 and +6 (for example, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W or Th) in the original reagents, or adding the necessary amounts to the scintillation material, inhibits generation of Ce4+ ion due to charge compensation, thus not only improving the scintillation property of the crystal but also reducing cracking and preventing formation of vacancies in the oxygen sublattice. Thus, when an ion with an oxidation number of +4, +5 or +6 (for example, Zr, Sn, Hf, As, V, Nb, Sb, Ta, Mo, W or Th) is present in the original reagent, or a necessary amount thereof is added to the scintillation material, a satisfactory scintillation property is obtained as a result, even when using a cheaper starting material of low purity containing any of the aforementioned 50 or more elements as impurities.
Japanese Unexamined Patent Publication No. 2006-199727 describes a Ce and Tm co-activated lutetium silicate single crystal as a cerium-activated lanthanoid silicate scintillator single crystal represented by the general formula Ce2xLn2yLu2(1−x−y)SiO5 (where Ln is any element from among lanthanoid elements except for Lu, and 2×10−4≦x≦3×10−2, 1×10−4≦y≦1×10−3), and it is stated that coactivation with Tm improves variation in light output, decay time and energy resolution.
Also, Japanese Unexamined Patent Publication No. 2005-350608 discloses a Ce and Ti co-activated gadolinium silicate single crystal as a rare earth silicate scintillator single crystal, wherein the parent material is a rare earth silicate crystal, the rare earth element is selected from the group consisting of Sc, Y, La, Gd and Lu, and Ti and Ce are included as luminescent center elements preferably with a Ti/Ce molar ratio of 1/10,000-1/10, and it is stated that coactivation with Ti improves the light output and speeds the decay time.
In Japanese Unexamined Patent Publication No. 2007-2226 there is described a Ce-activated gadolinium lutetium silicate (Lu composition: 20%) single crystal with an emission intensity spectrum with a maximum peak wavelength in the range of 450 nm-600 nm, as a scintillator single crystal having a chemical composition represented by the general formula: CexLnySizOu (where Ln represents at least two different elements selected from among Y, Gd and Lu, and 0.001≦x≦0.1, 1.9≦y≦2.1, 0.9≦z≦1.1, 4.9≦u≦5.1).
Finally, Japanese Unexamined Patent Publication No. 2006-257199 describes a Ce-activated gadolinium lutetium silicate (Lu composition: 20%) single crystal characterized by having an emission intensity peak wavelength of longer than 450 nm and a half-width of greater than 112 nm, upon excitation with any wavelength 360 nm-400 nm, as a rare earth silicate scintillation material represented by the general formula: Ln2xGd2(1−x−y)Ce2ySiO5 (where Ln represents at least one element selected from among Sc, Y and Lu, and 0.1≦x≦0.5, 0.01≦y≦≦0.1).